The incidence and nature of intranuclear inclusions found in tumor cells of preoperative and postoperative cytologic, histologic and electron microscopic (EM) specimens were studied in 38 cases of papillary adenocarcinoma of the lung in order to assess the cytodiagnostic significance of these inclusions. The overall incidence of the intranuclear inclusions was 34%; the incidence was higher in the well-differentiated type of adenocarcinoma (48%). Four smear and four EM types of inclusions were recognized. Cytoplasmic invaginations (pseudoinclusions), single-membrane-bound inclusions, aggregates of 50-nm tubules and any combination of the above could be distinguished by EM. There was some correlation between the smear and EM types of inclusions. EM analysis suggests that intranuclear inclusions, except for pseudoinclusions, may be derived from the inner nuclear membrane. In routine cytology, the intranuclear inclusion is a good diagnostic marker for papillary adenocarcinoma of the lung.